This invention relates to electronic ballasts for gas discharge lamps and, in particular, to a compact ballast for instant start fluorescent lamps.
A fluorescent lamp is a non-linear electrical load, i.e. the current through the lamp is not proportional to the voltage across the lamp. The current is zero until the voltage increases sufficiently for an arc to strike, then the current will increase rapidly through the ionized gases in the lamp unless there is a ballast in series with the lamp to limit current.
In some fluorescent lamps, small filaments at each end of the lamp are made to glow and emit electrons to facilitate starting the lamp. Lamps without these filaments, or "heaters," are called "instant start" lamps because there is no delay while the filaments are heated. An instant start lamp must be started at a higher voltage and current than a lamp with heaters, which requires that the electronic ballasts for such lamps be more powerful and, to some extent, more dangerous.
A "magnetic" ballast is an inductor in series with a lamp for limiting current through the lamp. The inductor includes many turns of wire wound on a laminated iron core and magnetic ballasts of the prior art are physically large and heavy, often accounting for more than half the weight of a fixture including the lamps.
An "electronic" ballast typically includes a converter for changing the AC from a power line to direct current (DC) and an inverter for changing the DC to high frequency AC. Converting from AC to DC is usually done with a full wave, or bridge, rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a "boost" circuit to improve power factor or to increase the voltage on the filter capacitor from approximately 140 volts to 300 volts or higher (from a 120 volt AC input). The inverter changes the DC to high frequency AC at 140-300 volts for powering one or more fluorescent lamps.
Because electronic ballasts operate at a higher frequency than a power line (e.g. 30 khz compared to 50/60 , hz), the "magnetics" in an electronic ballast are much smaller than the inductor in a magnetic ballast. Despite the smaller inductors, an electronic ballast is capable of delivering a significant amount of power, at least for a short time, in order to start an instant start lamp. Therein lies a problem because, if a lamp is defective or missing, a ballast for an instant start lamp can produce a significant arc and may cause a fire.
An electronic ballast is not intended to be operated without a lamp. If a lamp is not connected to the ballast, or if a lamp is defective, then the voltage on the sockets for the lamp can greatly exceed 300 volts. This creates a potentially hazardous situation for anyone who may come into contact with a socket or who may be near the arc created by ballasts of the prior art.
One solution to this problem is to use a transformer for coupling power to a lamp, thereby isolating the sockets from ground and from the fixture for the lamp. An output transformer is undesirable for reasons of size, weight, and cost, even for a transformer operating at the higher frequency of an electronic ballast.
U.S. Pat. No. 5,500,576 (Russell et al.) discloses a very compact ballast operating at high efficiency, excellent power factor and including fault detection circuitry. That ballast, as described, can not operate with instant start lamps or operate other lamps in instant start mode. An instant start lamp requires a large impulse of energy to start and such an impulse from the patented ballast would be sensed as a fault, turning off the ballast.
Instant start electronic ballasts have become a low cost means for ballasting what are known as T8 lamps, even though these lamps include heaters. Most instant start ballasts include a current fed, push-pull, parallel resonant circuit. While this circuit is relatively simple to design and produce, it has several disadvantages. One disadvantage is the relatively large current circulating in the resonant circuit. While useful for starting an instant start lamp, the circulating current leads to power losses and low efficiency. Another disadvantage of conventional instant start ballasts of the prior art is the bulky output transformer. A further disadvantage is that, when lamps start rectifying at the end of their life, the heaters can glow as brightly as the filament in an incandescent lamp, producing very high temperatures.
It is known in the art to provide an electronic ballast having a direct coupled output, in which a lamp is connected in parallel with the capacitor in a series resonant LC circuit. Such ballasts require additional circuitry to sense fault conditions, such as a missing or defective lamp, and to shut off the ballast. A problem with fault detection circuitry is the power consumed when the lamp is operating normally, i.e. adding fault detection circuitry can decrease the efficiency of a ballast. Another problem with fault detection circuitry is that it is difficult to tell the difference between a fault condition and normal starting in an instant start lamp.
U.S. Pat. 5,574,336 (Konopka et al.) discloses a ballast for a fluorescent lamp in which an inverter is turned on or off by a control circuit that is turned on or off by a timing circuit that is coupled to the lamp. If lamp current is not detected within a short period of starting, the timing circuit turns off the control circuit, which turns off the inverter. If lamp current is detected, then the inverter continues to run and provides power to the lamp and to the control circuit.
In view of the foregoing, it is therefore an object of the invention to provide an improved compact ballast for instant start lamps and for lamps with heaters but operated as instant start lamps.
Another object of the invention is to provide an instant start ballast having more efficient operation and improved fault detection.
A further object of the invention is to provide an instant start ballast that does not have a transformer output or a parallel resonant circuit.
Another object of the invention is to provide an instant start ballast that can suppress in a few hundred milliseconds, or less, an arc external to the lamp due to a faulty connection.
A further object of the invention is to provide an instant start ballast that can distinguish between a fault and normal starting.
Another object of the invention is to provide an instant start ballast that automatically reduces power at the end of the life of the lamp.